The use of ink-jet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today's ink-jet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago.
An ink-jet image is formed when a precise pattern of dots is ejected from a drop generating device known as a "printhead" onto a printing medium. The typical ink-jet printhead has an array of precisely formed nozzles attached to a thermal ink-jet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a "firing resistor," located opposite the nozzle so ink can collect between the firing resistor and the nozzle. The printhead is held and protected by an outer packaging referred to as a print cartridge.
The print cartridge is mounted on a carriage that travels along the width of the printer. As the print medium and the print cartridge are positioned in the desired location in the printer, the appropriate resistor is electronically enabled. When electric printing pulses heat the thermal ink-jet firing resistor, a small volume of ink adjacent the firing resistor is heated, vaporizing a bubble of ink, and thereby ejecting a drop of ink from the printhead. The drops strike the printing medium and then dry to form "dots" that, when viewed together, form the printed image. A "pass" is completed once the carriage has spanned the entire width of the medium. A single image element (pixel) can receive one or more drops of ink during a single pass or multiple passes from the same or different nozzles. The number of passes depends on many factors such as the print application and choice of print medium. Examples of print application include graphics or text. Examples of different print media include plain paper, coated paper, glossy paper, or overhead projector transparency film.
Color ink-jet printers typically use only three inks of differing hues, magenta, yellow, and cyan, and optionally a fourth achromatic black ink. The magenta, yellow, and cyan colors are referred to as subtractive "primary colors." Other "secondary colors" can be generated using different combinations of the primary colors by ejecting drops of the primary colors using either or both dot-on-dot (DOD) and dot-next-to-dot (DND) methods. In the DOD method mixing of the primary colors occurs when the drops are placed on top of one another on the print medium. In the DND method the mixing of colors occurs at the side by side or DND interface boundaries. The ejection of multiple ink drops occuring either during the same pass or different passes is well known in the art. For example, the secondary colors of red, blue, and green can be obtained by combining the following colors, respectively: magenta and yellow, red and cyan, yellow and cyan. The number of color combinations produceable (color gamut or gamut volume) differs for different printing systems. The volume of the color space (gamut volume) and the surface of the color space (gamut surface) are affected by the choice of the imaging system components such as dyes and media, and the imaging technique such as inkjet and photography.
Any given perceived color can be described using any one of the well known color spaces, such as CIELAB and Munsell. For example, in the Munsell System a given color is defined using three terms, Hue, Value, and Chroma. Similarly, as illustrated in FIGS. 1 and 2, in the CIELAB color space, a color is defined using three terms L*, a*, and b* and additional terms such as C* (chroma) and h.degree. (hue angle) are used to further describe a given color, wherein EQU C*.sub.ab =(a*.sup.2 +b*.sup.2).sup.1/2 equation 1 EQU h.degree..sub.ab =tan.sup.-1 b*/a* equation 2
One attribute detracting from the perceived quality of a printed image is undesirable granularity. Granularity refers to the non-uniformity and graininess of the image as perceived by the observer. Granularity is a function of both the dot size and contrast. Dot size refers to the size of an ink-jet droplet on the print medium and contrast refers to the measure of difference between two lightness levels--see graylevel, below. The smaller the dot size, the lower the granularity. The lower the contrast between two dots or between the dot and the white of the printed page, the lower the granularity.
In order to decrease granularity and to print sharper and more definite images, either or both the printer resolution and the image graylevel must be increased. Printer resolution is generally referred to as the number of dots per inch (dpi) the printer is capable of producing. Graylevel (or levels of gray) basically signifies the range of perceived lightness between black and white. Although the term graylevel originated for describing gray produced via black-only (achromatic) printing, it is also used to denote the equivalent in color printing (levels of color). The terms color level, levels of color, and color graylevel, all describe colors produced via color printing where the resulting colors having the same hue differ in either or both their Munsell Value and Munsell Chroma. Thus, here, the term graylevel is understood to refer to both levels of gray in black and white printing and levels of color in color printing. Graylevel can be increased through the use of dithering techniques in binary printers or depositing more than one level of lightness (or level of color) in gray level printers, as described below.
The drop on demand system and most of the continuous ink-jet printing systems are essentially binary, i.e., at each image element (pixel) of the paper there must be placed a droplet of ink or no ink at all, thus the term binary. The color white is produced by the paper itself. This binary performance limits the range of colors that can be produced with an ink-jet printer since a binary printing system cannot produce graylevels (or color levels) using only one pixel.
Well known dithering techniques in which the image is divided into very small square matrices (also referred to as "super pixels") have been used to increase the number of graylevels in a binary printer. Each matrix contains a certain number of pixels, such as 2.times.2 or 4.times.4 pixels. To produce the perception of gray, the black is applied spatially extending in two dimensions in differing quantities. This method is called halftoning and the resulting perception that grays are produced is called graylevels and is explained in U.S. Pat. No. 4,967,203 entitled "Interlace Printing Process" by Doan.
The term graylevel can also be used to indicate the ability of a printer (a graylevel printer) to deposit, unlike a binary printer, more than one level of lightness (or level of color) at a single pixel site. This differs from the use of the same term employed above for binary printers which require more than one pixel site to produce graylevels.
Different techniques utilizing graylevel printers have been used to increase color levels. One approach to varying color levels is to provide different colorant concentrations of an ink of a given hue. These different inks can be stored in separate ink-jet print cartridges, each with a separate ink reservoir; in different ink reservoir, within the same ink-jet print cartridge; or separate off-board ink reservoirs where the ink-jet print cartridges do not have self-contained ink reservoirs. However, these approaches require a number of ink reservoirs or print cartridges and thus require expensive, complex systems.
Another approach to varying graylevel is described in U.S. Pat. No. 4,494,128 entitled "Gray Scale Printing with Ink Jets" by Vaught. Vaught discloses a valving apparatus in a thermal ink-jet system for mixing clear ink vehicle (diluent) with the colored ink during the actual jet printing process to produce variation in visual print density (graylevel). Adapting such a system for color mixing in the firing chamber is suggested. Such techniques also require more expensive, and complex systems.
The use of white ink in some special applications has been described in patents such as U.S. Pat. No. 4,680,058 entitled "White Ink Compositions for Ink-Jet Printing" by Shimizu. The Shimizu patent, however, is limited to the use of white ink as an independent ink for printing on colored or dark print media.
Notwithstanding their recent success, intensive research and development efforts continue toward improving ink-jet print quality. In general, ink-jet print quality still falls short of that produced by more expensive technologies such as photography and offset or gravure printing. A surge in interest in ink-jet imaging (e.g., the rendition of pictures) has resulted in the need to produce near photographic quality printed images at a reasonable cost. The challenge remains to further improve the quality of ink-jet printed images without increasing their cost.
It will be apparent from the foregoing that although there are many processes and apparatus for improving the print quality of an ink-jet image, there is still a need for an approach that provides a cost effective and adaptable process for improving image quality without increasing printer resolution or relying on expensive and complicated apparatus.